High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of pyrrole

Abstract

The fragmentation dynamics of pyrrole molecules following excitation at many wavelengths in the range 193.3 o l phot o 254.0 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Excitation at the longer wavelengths within this range results in (vibronically induced) population of the 1 1 A 2 (ps*) excited state, but once l phot r 225 nm the electric dipole allowed 1 1 B 2 ' X 1 A 1 (p* ' p) transition becomes the dominant absorption. All of the total kinetic energy release (TKER) spectra so derived show a 'fast' peak, centred at TKER B7000 cm À1 . Analysis of the structure evident in this peak, particularly in spectra recorded at the longer excitation wavelengths, reveals selective population of specific vibrational levels of the pyrrolyl co-fragment. These have been assigned by comparison with calculated normal mode vibrational frequencies, leading to a precise determination of the N-H bond strength in pyrrole: D 0 ¼ 32850 AE 40 cm À1 , and the enthalpy of formation of the pyrrolyl radical: D f H 0 1(C 4 H 4 N) ¼ 301.9 AE 0.5 kJ mol À1 . The recoil anisotropy of the fast H atom photofragments formed following excitation to, and dissociation on, the 1 1 A 2 (ps*) potential energy surface (PES) is seen to depend upon the vibrational level of the pyrrolyl co-fragment. This observation, and the finding that the mean TKER associated with these fast H þ pyrrolyl fragments is essentially independent of l phot , can be explained by assuming that, upon N-H bond fission, the skeletal vibrational motions in pyrrole(1 1 A 2 ) molecules evolve adiabatically into the corresponding modes of the ground state pyrrolyl fragment. A second, 'slow' peak is increasingly evident in TKER spectra recorded at shorter photolysis wavelengths, and becomes the dominant feature once l phot r 218 nm. This component exhibits no recoil anisotropy; its TKER profile is reminiscent of that observed in many other dissociations that yield H atoms by 'statistical' decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at these shorter excitation wavelengths is rationalised by assuming two possible decay routes for pyrrole molecules excited to the 1 B 2 (pp*) state. One involves fast 1 1 B 2 * 1 1 A 2 radiationless transfer and subsequent fragmentation on the 1 1 A 2 PES, yielding 'fast' H atoms (and pyrrolyl co-fragments)-reminiscent of behaviour seen at longer excitation wavelengths where the 1 1 A 2 PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, either by successive 1 1 B 2 * 1 1 A 2 * X 1 A 1 couplings mediated by conical intersections between the relevant PESs or, possibly, by an as yet unrecognised direct 1 1 B 2 * X 1 A 1 coupling, and subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding 'slow' H atoms (together with, most probably, cyanoallyl co-fragments).

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

“…This is also in contrast to pyrrole, where anisotropic recoil of fast H-atom fragments has been observed. 66,67 Following the population of the 1 πσ* state at 249 nm the indole molecule may undergo dissociation along the N-H bond to yield an H atom and the ground state indolyl radical, as inferred from the previously discussed fast kinetic energy channels reported in studies probing either the indolyl or H atom photoproducts. 29,30 A second, alternative route is non-radiative relaxation to the vibrationally excited ground electronic state of indole.…”

UV Photodissociation of Pyrroles: Symmetry and Substituent Effects

“…13,14 Higher resolution TKER spectra of the H + pyrrolyl fragments formed following near UV excitation to the S 1 state of pyrrole show structure attributable to population of specific vibrational levels of the radical product. 15,16 The relative branching into these levels varies with photolysis wavelength, but all of the populated levels (apart from the v = 0 level) are found to involve non-totally symmetric nuclear motions. The product energy disposals and recoil anisotropies have been rationalised in terms of vibronically induced S 1 S 0 excitation, followed by prompt N-H bond fission -a view confirmed in a number of ultrafast pump-probe studies.…”

“…the out-of-plane NH wag (b 1 symmetry in C 2v )) also promotes coupling to a higher electronic state. 15 substituents respectively withdraw electron density from, and donate electron density to, the ring π-system. Each is shown to have a distinctive and, to a large extent, predictable effect on the parent excitation spectrum and the ensuing fragmentation dynamics.…”

Femtosecond Multiphoton Ionization of Pyrrole

“…37 Other vibrational modes, for example, ν 7 (1092 cm À1 ), may have also contributed to the intensity, which had almost the same vibrational energy as ν 8 . 22 Our 1 + 1 two-photon ionization experimental results revealed that the excitation of the pyrrole cation had an internal energy of 0.13 eV (1048 cm À1 ), which was due to the vibrational excitation of mode ν 8 or a superposition of transitions to of modes ν 8 and ν 7 .…”

“…Two decay mechanisms from the 1A 2 state have been proposed: direct dissociation from the initial 1A 2 state and statistical decay following internal conversion to the ground electronic state. 22 Photoelectron spectroscopy (PES) has been used as an invaluable tool for studying the electron structure of molecules for a long time. PES measurements of pyrrole have been obtained using different methods.…”